The present invention relates to a positive electrode active material that is made of a Li-containing transition metal composite oxide and used for a nonaqueous electrolytic secondary battery, and a nonaqueous electrolytic solution secondary battery therewith.
Recently, note type personal computers, personal digital assistants (PDAS), cellular phones and soon are rapidly spreading. As they spread, there is a strong demand in that multi-functional portable electronic instruments are allowed to operate for a longer period. Accordingly, there is an ever stronger demand for smaller and higher capacity secondary batteries, which may be used as power source of various kinds of electronic equipment including portable electronic equipment.
As a secondary battery capable of satisfying such demands, there is known a Li ion secondary battery that utilizes a nonaqueous electrolytic solution including, for instance, a Li salt. In the Li-ion secondary battery, a Li containing transition metal composite oxide, such as LiCoO2 and LiNiO2, is used as the positive electrode active material. For a negative electrode, carbonaceous material is utilizes, and a nonaqueous electrolytic solution, in which a lithium salt, such as LiPF6 or LiBF4, is dissolved in a nonaqueous solvent, is utilized. Such Li ion secondary batteries are in heavy usage as a power source or the like of the portable electronic equipment.
The positive electrode active material, such as LiCoO2 or LiNiO2, is normally obtained by sintering a mixture of cobalt oxide or nickel oxide and lithium carbonate in air at a temperature of substantially 900xc2x0 C., and thereby converting into a composite oxide. In the Li-ion secondary battery, the positive electrode active material largely affects on the battery characteristics and so on. Accordingly, in order to improve the battery characteristics and to overcome problems in manufacture of the positive electrode active material, various additives to LiCoO2 or LiNiO2 have been proposed.
For instance, Japanese Patent Laid-Open Application No. 62-90863 JP-A discloses an active material expressed by AxMyNzO2 (wherein A denotes alkali metal element, such as Li or the like, M denotes a transition metal element, such as Co, Ni and Mn, N denotes at least one kind element selected from Al, In and Sn, and 0.05xe2x89xa6xxe2x89xa61.10, 0.85xe2x89xa6yxe2x89xa61.00, 0.001 xe2x89xa6zxe2x89xa60.10). In this, the battery characteristics, such as charge-discharge cycle characteristics, are improved by use of an additive, such as Al, In, or Sn.
Japanese Patent Laid-Open Application No. 63-121258 JP-A discloses a positive electrode active material thereto Sc, Mn, Ti, Rb, Sr, B, or P is further added. Furthermore, Japanese Patent Laid-Open Application No. 10-1316 JP-A discloses a positive electrode active material in which Co in LiCoO2 is partially replaced by an element, such as B, Mg, Si, Cu, Ce, Y, Ti, V, Mn, Fe, Ni, Sn, or Zr.
In order to improve the rate characteristics and temperature characteristics of the lithium ion secondary battery, particle size of the positive electrode active material is preferable to be finer. In particular, in order to improve low temperature characteristics of the battery, it is considered that miniaturization of the particle size of the positive electrode active material is indispensable. The particle size of the positive electrode active material is generally controlled through the sintering temperature. Specifically, it has been tried to make the particle size finer by sintering at a temperature of substantially 800xc2x0 C.
However, in the positive electrode active material, which is sintered at low temperatures, because of insufficient reaction, there is a problem in that it deteriorates charge-discharge cycle characteristics of the secondary battery. This is because LiCoO2 structure grows insufficiently due to the low temperature sintering. Furthermore, the finer particle size may be obtained also by making a Li/Co ratio of the LiCoO2 active material smaller. However, in this case, since, similarly as the case of the low temperature sintering, the sufficient crystallinity may not be obtained, battery capacity and charge-discharge cycle characteristics deteriorate.
Furthermore, in order to control the particle size of the positive electrode active material, it has been tried to add various kinds of elements. However, the added elements, when used as an actual secondary battery, may cause a likelihood of such as gas evolution or precipitation due to repetition of charge and discharge. As a result, pressure inside the battery rises, and, in extreme cases, there is a problem in that a pressure valve of the battery operates, and the secondary battery could be destroyed.
An object of the present invention is to provide a positive electrode active material for a nonaqueous electrolytic solution secondary battery that, while maintaining the controllability (miniaturization control) of the particle size, may allow suppressing the gas evolution or the like. Another object of the present invention is to provide, by use of such positive electrode active material, a nonaqueous electrolytic solution secondary battery that may allow improving battery characteristics, such as charge-discharge cycle characteristics and temperature characteristics, and inhibiting the pressure rise in the battery.
A positive electrode active material of the present invention for nonaqueous electrolytic solution secondary batteries is substantially made of a Li containing transition metal composite oxide, which has a composition expressed by,
General formula: LixMySnzO2
(wherein M denotes at least one kind element selected from transition metal elements, and x, y and z denote the numbers satisfying, respectively, 0.9 xe2x89xa6xxe2x89xa61.15, 0.85xe2x89xa6yxe2x89xa61.00, and 0m less than z less than 0.001). In the positive electrode active material of the present invention for nonaqueous electrolytic solution secondary batteries, Co may be preferably used as at least part of the M element.
The nonaqueous electrolytic solution secondary battery of the present invention includes a positive electrode, which contains the aforementioned positive electrode active material of the present invention for nonaqueous electrolytic solution secondary batteries: a negative electrode disposed to face the positive electrode through a separator: a battery case for accommodating the positive electrode, the separator, and the negative electrode: and a nonaqueous electrolytic solution filled in the battery case.
The positive electrode active material of the present invention for nonaqueous electrolytic solution secondary batteries contains a very slight amount of Sn (in the range of 0 less than z less than 0.001 as a z value in the general formula). Some elements have been known as the additive, which is capable of controlling so that the particle size of the positive electrode active material, such as LiCoO2 and so on, may be fine. Among these, it is found that Sn, in particular, may sufficiently exhibit the effect by an only slight addition.
That is, the present positive electrode active material containing a very slight amount of Sn may allow making the particle size finer by sintering under ordinary conditions. Furthermore, a more sharp particle size distribution may be obtained. Thereby, the battery characteristics, such as charge-discharge cycle characteristics and temperature characteristics may be improved.
In addition to the above, since the Sn, which exhibits the aforementioned effect, is contained by an only very slight amount, the pressure inside the battery may be suppressed from rising. Mechanism of gas evolution when the Sn is present is not sufficiently elucidated. However, gasification of the electrolytic solution due to a catalytic action of the Sn may be considered. In the present invention, since the Sn is only slightly added, the catalytic action of the Sn may be suppressed, thereby the gasification of the electrolytic solution may be inhibited from occurring.